Support relative paths

[khaledgabr77]

Currently the config files from package support absolute path.
ex: world_path: "/home/khaled/riout_ws/src/ROSNavBench/simulations/worlds/turtlebot3_house_gz.sdf"

we need to update files that need updates to support relative paths.

ex: world_path: "ROSNavBench/simulations/worlds/turtlebot3_house_gz.sdf"

Files that need to be updated:

• launch/main.launch.py

• launch/nav2.launch.py

• launch/spawn_robot.launch.py

• ROSNavBench/trajectory_generator.py

• ROSNavBench/pdf_generator.py

• ROSNavBench/follow_path.py